1. Field of Invention
The field of the currently claimed embodiments of this invention relates to systems and methods for sensing corneal hydration, and more particularly to systems and methods for sensing corneal hydration using terahertz illumination.
2. Discussion of Related Art
Corneal hydration is currently approximated in the clinic by extrapolation using the central corneal thickness (CCT) measurements. These methods are based on ultrasound or optical pachymetry and assume a linear relationship between the CCT and the average water content of the eye. This relationship was established from the empirical fit of 11 healthy human corneas from a cornea bank. No correlation statistics are given, but deviations of 20% or greater are seen in the data, and the linear fit predicts a dehydrated corneal thickness of ˜90 micrometers, when values of ˜200 micrometers are reported in the literature. Furthermore, the extrapolation is based upon healthy corneas and this approach also cannot provide accurate analysis for changes relating to disease states in cornea. Even if these fits could be improved by increasing the number of samples, extrapolation of corneal hydration from CCT based on samples from the population cannot account for physiological variations between people, including geriatric and pediatric cases. An additional complication to the current state of the art is that most clinical methods for measuring corneal thickness in vivo are themselves modified by changes in hydration. Hydration changes in the cornea significantly modify the speed of sound and the refractive index of the tissue thus further reducing the accuracy of ultrasound and optical pachymetry.
Remote measurement of corneal hydration has also been demonstrated using confocal Raman spectroscopy, which can study the OH and CH bonds in a material by illuminating it with monochromatic laser radiation and collecting scattered light. However, the scattering yield is typically 10−6 to 10−8 and the excitation illumination fluence necessary to achieve accurate measurements exceeds the ANSI regulations for use in humans by orders of magnitude.
Therefore, pachymetry methods offer very accurate thickness measurements but the mapping from thickness to hydration is very inaccurate. Thus ophthalmologic pachymetry is limited by inherent constraints and no amount of system or methodological improvements can overcome these limitations. Therefore, there remains a need for improved systems for sensing corneal hydration.